In a multichip module (MCM), at least two semiconductor chips are stacked together on a package substrate. An upper chip located at an upper side is, for example, flip-chip mounted on a lower chip located at a lower side. Typically, flip chip technology refers to a method for interconnecting semiconductor devices, such as IC chips, to external circuitry (e.g., the package substrate) with conductive bumps that have been deposited onto chip pads. Conductive bumps provided for the upper chip are placed on conductive pads provided on the lower chip. The conductive bumps provided for the upper chip are bonded in advance to conductive pads provided thereon. The conductive bumps are formed, for example, of gold (Au). The conductive pads are formed, for example, of aluminum (Al). Between the upper chip and the lower chip, an underfill material is filled. By the function of the underfill material, the upper chip is fixed to the lower chip (for example, see Japanese Laid-Open Patent Applications 2007-129220, 7-169797 and 2001-85470).
In the multichip module, gold of the conductive bump diffuses into aluminum of the conductive pad. Accordingly, an intermetallic compound is formed between the gold and the aluminum. When the intermetallic compound grows and comes in contact with, for example, adjacent conductive pads, short circuit occurs between the conductive pads. In addition, because of the difference in the diffusion rate between gold and aluminum, voids, which are called Kirkendall voids, are formed between the conductive pad and the conductive bump. As the voids are enlarged, an increase in resistance and an electrical disconnection unfavorably occur between the conductive pad and the conductive bump.